 INES 1 |
Potential Loss of Containment Due to Small Crack in Torus |
On June 27, 2005 at the James A. FitzPatrick plant, inspectors identified a small leak coming from a through-wall crack of the torus shell. The torus crack size was about 4.6 inches long, but with some branching cracks connected to the major crack. The crack at FitzPatrick was located close to an external support column which is welded to the outside of the torus. These external support columns were added in the 1980s as part of the Mark I improvements to stiffen the torus against blowdown stresses. The torus wall is 5/8 inch thick.
The licensee originally did not declare Primary Containment inoperable because of the crack. Once the licensee identified that the crack was located about 5 feet below the waterline and just below the HPCI exhaust pipe discharge, it declared Primary Containment inoperable and initiated a normal plant shutdown. In addition, the licensee declared a Notification of Unusual Event (NOUE) in accordance with Emergency Action Level 9.1.2, for the “Potential Loss of Containment.”
The plant’s High Pressure Coolant Injection (HPCI) exhaust line, although consistent with original General Electric design specifications, does not include a HPCI turbine exhaust line sparger. A properly designed sparger is expected to reduce local torus shell stresses resulting from HPCI turbine exhaust pressure pulses. The licensee determined that the torus shell experienced localized stress, high cycle fatigue due to rapid condensation of the HPCI exhaust steam, which led to the torus crack.
As part of the corrective actions, the licensee performed an ASME Code repair on the torus shell. The licensee plans to perform accelerated non-destructive examination on the high stress areas of the torus shell in the vicinity of the HPCI and Reactor Core Isolation Cooling (RCIC) system turbine exhaust lines, following turbine operation. In addition, the licensee will modify the HPCI exhaust line and ring girder gusset attachment as required to reduce the associated stresses.
FitzPatrick had not previously installed a HPCI sparger due to inadequate information transfer from GE and other facilities regarding concerns with pressure instability and vibration inside the torus. The licensee identified that GE was aware of this issue as early as 1972. Because of prior deficiencies in the sharing of operating experience, FitzPatrick was not aware of the concerns.
Event date: Mon, 27-06-2005
